Ever since it started, seems there no end to it. How long does these stupid waves run? I think I kill off like 7 portal so far and they keep respawn. I’m getting pissed here. Is this endless and I’m supposed to guard these spots forever?

It's not endless. But on Very High-Alien-Aggression and with a lot of strong Factions on the map the invasion will be longer and the portals respawn several times.
This gets worse when there's less space for them to make sense spawning.

With Medium AI, Medium Aliens, Huge world, Marathon pace, there was 5 portals (only one at home), and I think none respawned. In the end there was a floating mothership at my coast, that I easily finished off (it was coming from other factions seas and had only few % left).